Method of polishing glass

ABSTRACT

A glass workpiece is polished to obtain a surface having a high luster and a substantial absence of polishing defects by applying to the surface a slurry of a glass powder of substantially the same hardness and composition as the workpiece and a liquid carrier. The slurry also contains a buffering agent which stabilizes its pH between 8 and 12 and preferably between 10 and 11. Its specific gravity preferably lies between 1.14 and 1.19. A rotating polishing pad made of a synthetic fiber is then pressed against the workpiece to polish the piece.

United States Patent [1 3 673 746 Highberg July 4, 1972 54] METHOD OFPOLISHING GLASS 3,404,488 10/1968 Cox ..s1/2s4 [72] Inventor: Cafle w.Highberg Sylvania Ohio 3,587,196 6/1971 Dunn ..5l/283 X [73] Assignee:Engelhard Hanovia, lnc., Newark, NJ. i y mmi rD0nfl|d K lly Feb 3 1971AttorneyBrumbaugh, Graves, Donohue & Raymond 21 Appl. No.: 112,428 [5 1B RAC A glass workpiece is polished to obtain a surface having a high 52us. Cl ..51/283 luster and a Substantial absence of Wishing defects byapply- [51] Int. Cl ..B24b 1/00 ing to the Surface a Slurry ofa glassPowder of Substamiany the 581 Field of Search ..5 l/283 284 3023 samehardness and compsifin as and a quid carrier. The slurry also contains abuffering agent which stabil- 5 f izes its pH between 8 and 12 andpreferably between 10 and l 6] Re erences Cited 11. Its specificgravity'preferably lies between 1.14 and 1.19. UNITED STATES PATENTS Arotating polishing pad made of a synthetic fiber is then 2 000 240 /1 5L 51/283 pressed against the workpiece to polish the piece.

, ong

Touvay ..5l/283 14 Claims, No Drawings METHOD OF POLISHING GLASSBACKGROUND OF THE INVENTION The present invention relates to methods ofpolishing glass, and more particularly to a novel and highly effectivemethod of polishing glass in which glass powder is used as an abrasive.

According to conventional methods of producing a glass article having ahighly polished surface, such as those desired on mirrors and televisionfaceplates, a glass workpiece is first subjected to a grinding process.Grinding is accomplished using a relatively large grained loose abrasivesuch as sand or a diamond or cubic boron nitride grinding wheel wherethe abrasive particles are fixed in a rigid matrix.

After grinding, the workpiece is conventionally subjected to finepolishing with a relatively fine grained abrasive such as cerium oxide,zirconium oxide or iron oxide. The fine grained abrasive forms part of aslurry that is applied to the coarse ground surface. The workpiece isthen conventionally held on a polishing table which reciprocates orrotates as it is fed past a rotating polishing pad formed of naturalfelt. This type of operation is often carried out using avertical-spindle reciprocating or rotary lapping machine.

It has also been proposed that more than one step fine grinding orsmoothing be carried out, employing mixtures of abrasives includingsubstances such as quartz, sand, glass and porcelain. It has beenfurther proposed in US. Pat. No. 2,000,240 that particles of groundglass could be utilized for an intermediate fine grinding step of theoverall process.

SUMMARY OF THE INVENTION The process of this invention is an improvementof considerable significance in the art of polishing glass. By thisprocess, a higher quality product can be obtained more simply,economically and at a high rate of productivity. A surface may beproduced which is substantially free of polishing defects and has anunusually high luster.

The process uses as an abrasive a glass powder of substantially the samehardness as the workpiece. The powder is suspended in a liquid carrierto form a slurry. The carrier may be ordinary tap water. The pH of theslurry is maintained at relatively high alkaline level by the inclusionof a bufiering agent. If the abrasive particles are of a relativelylarge size, a suspending agent may be incorporated in the slurry.

The glass powder may be, and preferably is, of the same composition aswell as the same hardness as the workpiece. The need to periodicallyreplenish the abrasive supply is therefore eliminated to some extentbecause the glass removed during the polishing operation can be allowedto become part of the slurry and thereby replenish the abrasive supply.It is thus always possible to use the same slurry for several successivepolishing operations. In some cases more abrasive material is formedthan is used up during successive polishing operations, and in thesecases successive polishing operations can be carried out indefinitelywithout further abrasive replenishment. Accordingly, the expense ofadded abrasive is reduced or eliminated and it is not necessary tofrequently interrupt the polishing operation while the abrasive supplyis replenished.

The abrasive required to initiate the polishing operation is readily andeconomically available as a byproduct of the grinding operation whichprecedes the polishing only when fixed abrasive (such as diamond)grinding is employed.

The polishing operation of the invention is most effectively carried outwhen the pH of the polishing slurry lies between approximately 8 and 12.The best results are generally obtained when the pH lies between 10 andll. The pH is stabilized at the desired alkaline level by incorporatinga bufiering agent in the slurry. Any of a large number of knownbufiering agents, such as a mixture of sodium hydroxide and boric acidcould be used. It is highly preferable to use polishing pads made ofsynthetic fibers, such as nylon, rather than natural fibers, such aswool, because synthetic fibers are better able to withstand therelatively high pH of the slurry.

The specific gravity of the slurry preferably lies between approximately1.14 and 1.19 and the preferred size of the glass particles lies betweenapproximately 3 and 50 microns.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the invention, ahighly polished glas surface is obtained by first subjecting a glassworkpiece to a conventional grinding operation, such as grinding with afixed abrasive such as, but not limited to, a diamond wheel. The finalfine polishing of the glass surface is then begun. An example of asuitable workpiece which is ready to be fine polished and from which theground glass powder may be obtained during the grinding process is asoda-lime glass plate of the type used for color television faceplateshaving a surface roughness of approximately 3.5 to 5.0 microinches(root-meansquare) as measured by a profilometer.

A slurry consisting of such glass powder and a carrier comprisingordinary tap water is applied to the ground surface. It is not necessaryto add any other abrasive to the slurry insofar as glass powder of thesame hardness and composition as the workpiece is preferred in theinstant invention. It is, therefore, possible to utilize the glasspowder removed during the grinding operation as an abrasive to initiatethe polishing operation. Glass particles removed from the workpieceduring the polishing process become part of the abrasive component ofthe slurry and permit the process to be continued without replenishmentof the abrasive supply from another source.

The pH of the slurry has a very significant effect on the glass removalrate and the quality of the surface. Substantially higher pH values thanthose present during conventional polishing operations are preferred.Slurries having pH values ranging from approximately 8.0 to 12.0 yieldgood results, with the optimum pH range lying between approximately 10.0and 11.0.

The pH of the slurry is stabilized at the desired high level by theaddition of a conventional buffering agent.'The presence of thebuffering agent causes the slurry to resist changes in its ionconcentration upon the addition of an acid or an alkali. If it were notfor the buflering agent the pH of the slurry would tend to be much lowerand would become slightly more alkaline as the process continued. Thiswould be undesirable because the effectiveness of the process isdiminished if the pH of the slurry departs from the preferred range andbecause the continuous change which would otherwise occur in the pHwould cause the process and the product to take on a nonuniformcharacter.

An example of a suitable buffering agent is a mixture of sodiumhydroxide and boric acid. A slurry containing this buffering agent canbe prepared by dissolving 4 gms of sodium hydroxide in 1,000 ml of waterto form a first solution and dissolving 6.2 gms of boric acid in 1000 mlof water to form a second solution. A quantity of 440 ml of the firstsolution is then combined in 500 ml of the second solution and 60 ml ofplain water. The resulting slurry has a pH of 10.5 which is highlydesirable.

The size of the glass particles when a batch is first preparedpreferably ranges from about 3 to 50 microns with the optimum glasspowder size range being approximately 5 15 microns. When the glassparticle size is less than approximately 15 microns, an adequatesuspension can readily be obtained 7 without using a suspending agent.When larger particles are used, however, it is generally preferable toinclude a suspending agent, such as plaster of paris, in the slurry.Larger glass particles (25 to 50 microns) have the advantage ofproviding a higher rate of glass removal without significantly affectingthe quality of the surface produced.

The thickness of the slurry also affects the glass removal rate and thesurface quality. The best results can be obtained using a slurry havinga specific gravity which lies between approximately 1.14 and 1.19. Thisis substantially thicker than a conventional abrasive slurry.

It has been found that polishing pads of the type conventionally used topolish glass are unsuitable for use in the process of the invention.Conventional pads result in an inferior surface, a low rate of glassremoval, and have a short pad life. This is because conventional padsare made of natural fibers which are adversely affected by therelatively high pH solutions that are used in this process. The presentinvention is practiced using pads made of synthetic fibers. Good resultsare obtained using pads made of polypropylene fibers. An even better padmaterial is nylon which can be used in the form of a felt or acarpet-type material. Felt yields somewhat more desirable results and isadvantageous because it is commercially available in thicknesses of upto one-half of an inch. The thickness of the pad is important because itcontributes to pad life, thereby affecting the down time required forpad replacement.

Under equal operating pressures applied to the polishing pad, glasspowder develops relatively little drag in comparison to conventionalpolishing abrasives such as cerium oxide. Because of this low drag, itis possible to increase the pressure on the polishing pad toapproximately two or three times that used with conventional abrasiveswithout increasing the horsepower required. Moreover, it is possible togreatly increase the pressure applied to the pad without developingvibrations in the mechanical polishing apparatus. Thus, because of thelow drag, operating pressure can be increased to pounds per square inch,for example, at which pressure the glass removal rate obtained usingglass powder exceeds that which can be obtained using conventionalbatches of polishing abrasives.

Productivity is increased and a superior polished surface can beobtained using the glass powder polishing process described herein,particularly at higher operating pressures. Possibly'this surprisingobservation is due to a healing effect on the surface of the workpiece,i.e., the glass near a surface defect is welded or fused tending tosmooth and fill the defect. This healing efiect then might reduce thequantity of glass which must be removed to obtain the highly polishedsurface desired. This observation is demonstrated by the following testscarried out on 2-inch diameter circular discs of soda-lime glass withequivalent diamond grinding history. In Example I, cerium oxide, aconventional polishing abrasive, was used. In Example ll, glass powderwas used as an abrasive. Both processes carried out until grindingdamage could no longer be observed using a microscope having amagnification power of 100.

EXAMPLE 1 Cerium Oxide Polishing Pad Bausch 8t Lomb Optical PolishingPad Cerium Oxide Content of Slurry 1% by Wt. Pressure 23.5 lbs.Polishing Time 15 minutes Glass removal required to eliminate diamondgrinding damage l 100 Microinches.

EXAMPLE ll Glass Powder Polishing Pad Huyck 9000 C-2 Felt Glass PowderSize 13.0 Microns Average Spec. Grav. of Slurry l. l7 gnns./c.c. pH ofGlass Powder Slurry 10.5 suspending Agent 6.5% by Wt. Plate Glass StuccoPressure 34.5 lbs. Polishing Time l3 minutes Glass removal required tocompletely eliminate diamond grinding damage 775 Microinches.

When cerium oxide was used it was necessary to remove 1,100 microinchesof glass to eliminate the scratches produced by diamond grinding. Whenglass powder was used 775 mieroinches of removal was required. Bothworkpieces originally contained diamond grinding scratches having adepth of l rnicroinches.

The examples described above show that when the glass powder process isused, the same diamond grinding damage can be eliminated with theremoval of 30 percent less glass from the workpiece. Thus, even if theglassremoval rates are the same, and they can be higher when using glasspowder, productivity can be increased 30 percent using the process ofthe invention.

Because of the high polishing effect of the process, and because of theother advantages described above, a smoother surface of significantlyhigher quality can be obtained at a higher production rate as comparedto conventional processing. The superiority of this process is, in part,attributable to the higher rate of glass removal obtainable and, inpart,

due to the fact that not as much glass removal is required. Thesuperiority of this process is also partly attributable to theelimination of the need to replenish the abrasive supply because theglass removed from the workpiece becomes part of the slurry andreplenishes the abrasive component of the slurry.

It will be obvious to those skilled in the art that the abovedescribedembodiment is meant to be merely exemplary and that it is susceptible ofmodification and variation without departing from the spirit and scopeof the invention. Therefore, the invention is not deemed to be limitedexcept as defined by the appended claims.

I claim:

1. A method of polishing a glass workpiece comprising ap-- plying aslurry to the surface of the workpiece and pressing a moving polishingpad made of a synthetic fiber against the surface to polish theworkpiece, said slurry comprising an abrasive component consistingessentially of glass powder of substantially the same hardness as theworkpiece, a liquid carrier and a buffering agent, the pH of said slurrybeing maintained by said buffering agent between about 8 and 12.

2. The method of claim 1 wherein the pH of said slurry is maintained bysaid buffering agent between about 10 and l l.

3. The method of claim 1 wherein the glass powder is of the samecomposition as the workpiece.

4. The method of claim 3 wherein the glass workpiece is first subjectedto a grinding process and wherein the glass removed from the workpieceduring the grinding of the workpiece forms at least part of the abrasivecomponent of the slurry whereby the need for replenishment of theabrasive from another source is diminished.

5. The method of claim 1 wherein the liquid carrier is tap water.

6. The method of claim 1 wherein the particles forming the glass powderhave a size range between approximately 3 and 50 microns.

7. The method of claim 1 wherein the particles forming the glass powderhave a particle size range of between approximately 15 and 50 micronsand the slurry includes a suspending agent.

8. The method of claim 1 wherein the specific gravity of the slurry liesbetween approximately 1. l4 and 1.19.

9. The method of claim 1 wherein the workpiece is ground with a fixedabrasive wheel before the slurry is applied.

10. The method of claim 9 wherein the fixed abrasive wheel is a diamondwheel.

11. The method of claim 1 wherein the polishing pad is made of nylon.

12. The method of claim 1 wherein the polishing pad is made of nylonfelt.

13. The method of claim 1 wherein the workpiece is made of soda-limeglass.

14. A method of polishing the surface of a diamond ground glassworkpiece following the grinding of said workpiece comprising applyingthereto a slurry having a specific gravity ing polishing pad made ofnylon felt to the workpiece, the glass removed from the workpiece duringthe coarse grinding thereof forming at least a part of the slurrywhereby the polishing operation can be carried out without continuouslyreplenishing the abrasive from another source.

2. The method of claim 1 wherein the pH of said slurry is maintained bysaid buffering agent between about 10 and
 11. 3. The method of claim 1wherein the glass powder is of the same composition as the workpiece. 4.The method of claim 3 wherein the glass workpiece is first subjected toa grinding process and wherein the glass removed from the workpieceduring the grinding of the workpiece forms at least part of the abrasivecomponent of the slurry whereby the need for replenishment of theabrasive from another source is diminished.
 5. The method of claim 1wherein the liquid carrier is tap water.
 6. The method of claim 1wherein the Particles forming the glass powder have a size range betweenapproximately 3 and 50 microns.
 7. The method of claim 1 wherein theparticles forming the glass powder have a particle size range of betweenapproximately 15 and 50 microns and the slurry includes a suspendingagent.
 8. The method of claim 1 wherein the specific gravity of theslurry lies between approximately 1.14 and 1.19.
 9. The method of claim1 wherein the workpiece is ground with a fixed abrasive wheel before theslurry is applied.
 10. The method of claim 9 wherein the fixed abrasivewheel is a diamond wheel.
 11. The method of claim 1 wherein thepolishing pad is made of nylon.
 12. The method of claim 1 wherein thepolishing pad is made of nylon felt.
 13. The method of claim 1 whereinthe workpiece is made of soda-lime glass.
 14. A method of polishing thesurface of a diamond ground glass workpiece following the grinding ofsaid workpiece comprising applying thereto a slurry having a specificgravity which lies between approximately 1.14 and 1.19 which compriseswater, a suspending agent, an abrasive component consisting essentiallyof glass powder of substantially the same hardness and composition asthe workpiece having an average particle size between approximately 3and 50 microns, and a buffering agent, the pH of said slurry beingmaintained by said buffering agent between about 10 and 11, and applyinga moving polishing pad made of nylon felt to the workpiece, the glassremoved from the workpiece during the coarse grinding thereof forming atleast a part of the slurry whereby the polishing operation can becarried out without continuously replenishing the abrasive from anothersource.